Electrically powered vehicles such as hybrid electric vehicles and electric vehicles incorporate a secondary battery (rechargeable battery) for storing electric energy for use in driving the vehicle. As the charging and discharging of the secondary battery cause a temperature increase due to the internal resistance of the secondary battery, the secondary battery should be cooled down. To this end, there has been proposed an approach of providing a cooling fan near the secondary battery to forcedly air-cool the secondary battery using the cooling fan.
For example, JP 2010-158964 A discloses a cooling device for a vehicle's electrical components, wherein a cooling fan is provided near a secondary battery. In JP 2010-158964 A, a duct through which cooling air generated by driving the cooling fan flows is branched in two directions at a point of the duct, and the path of the cooling air is switched depending on the temperature of a charger mounted in a vehicle. The cooling device changes the command duty for the cooling fan in accordance with the temperature of a high-voltage secondary battery or a vehicle-mounted charger that is to be cooled. The cooling device detects whether or not a failure occurs in the cooling fan, and in the event of a failure, outputs a signal to a diagnosis system and changes the control of electrical components in accordance with the type of the failure.
Whether a malfunction occurs in the cooling fan is often determined based on a detected parameter such as an actual rpm (revolutions per minute) value of the cooling fan. Such determination should be performed under a steady state in which the cooling fan is driven stably. The reason is that significant fluctuations in driving conditions of the cooling fan (for example, the rpm of the fan) result in inclusion of an error in a detected parameter due to, for example, delays in control, and therefore impair the accuracy in determining whether or not a failure occurs.
In JP 2010-158964 A, as the command duty is changed in accordance with the temperature of a high-voltage secondary battery or a charger, it is unlikely for the cooling fan to be in a steady state, making it difficult to accurately determine whether or not a malfunction occurs in the cooling fan. To address this situation, the cooling fan may be driven at a constant duty (constant rpm) without changing the command duty (command rpm) for the cooling fan. In this case, because the driving conditions of the cooling fan are stable, whether or not a malfunction occurs can be determined accurately. However, if the cooling fan is continuously driven at a constant duty (constant rpm) regardless of the state of the secondary battery or vehicle's driving conditions, there are problems in that an insufficient rpm leads to degradation of the secondary battery, as it is not cooled sufficiently, and conversely, an unnecessarily high rpm causes excessive noise or power consumption.
In consideration of the above, an object of the present invention is to adequately cool the vehicle-driving secondary battery while providing a sufficient opportunity for detecting whether or not a malfunction occurs in the cooling fan.